In general, a thermal recording material (medium for thermal recording) is obtained by applying a coating liquid onto a support such as paper, plastic film and the like to form a heat coloring layer, wherein the coating liquid is obtained by grinding and dispersing a basic dye, which is colorless or in a pale color at ambient temperature, and an organic developer, to give respective fine particles, mixing them and adding a binder, a filler, a sensitizer, a lubricant, other additive and the like to the mixture. By heating with a thermal head, a thermal pen, a laser light and the like, it affords a developed color record. The basic principle of such recording method is considered to be the change of the dye to have a color upon chemical contact of an electron donative dye with an organic developer. Such recording method (thermal recording method) is characterized in that it is free of the need of complicated treatments such as development, fixing and the like, it can record in a short time using a comparatively economical and simple apparatus, is maintenance free, is free of noise during recording, and the obtained color is very clear, as compared to other recording methods conventionally put to practical use, and has been widely used as a recording material for computer output, printer of electronic calculator and the like, recorder for medical measurement, facsimile, automatic ticket vending machine, label fields, copying machine and the like. As multi-purpose and high performance types of these apparatuses have been progressively provided in recent years, a higher-grade thermal recording material (medium for thermal recording) has been demanded. For high speed recording and miniaturization of apparatuses, for example, the thermal energy of the thermal head of recording apparatuses tends to become very small, and a thermal recording material (medium for thermal recording) to be used therefor is required to show sufficient color density to afford a high density and clear color image even with a very little energy.
To meet such request, various compounds having phenolic hydroxyl group have been proposed as a developer to be contained in a heat coloring layer, and disclosed in, for example, JP-B-40-9309, JP-B-43-4160, JP-B-45-14039, JP-B-51-29830, JP-A-56-144193 and the like. Generally, bisphenol compounds, 4-hydroxybenzoic acid ester and the like have been put to practical use alone or in combination of several kinds thereof. However, the conventional materials such as these are associated with problems of, for example, low thermal response, insufficient color density achieved by high speed recording, inconsistent color densities, time-course changes in the density of color image after recording, discoloration during preservation, degraded heat resistance of background, precipitation of white powder on a surface, which is what is called blooming, degraded re-printability and the like.
There has also been disclosed recently a method using a trisphenol compound as a developer or an antifading agent in JP-A-9-278695, JP-A-2001-96926 and the like, but the use of such compound does not lead to a sufficient color density. While JP-A-58-181686 discloses a method using 2,2′-methylenediphenol compound as a developer, the method described in this publication is insufficient in sensitivity and image stability, such as heat resistance, moisture resistance, weather resistance and the like.
It is also known to add a novolac resin to a color layer of a thermal recording material, but this is associated with a problem that sufficient sensitivity cannot be obtained.
As mentioned above, a thermal recording material having preservation stability as mentioned above, such as heat resistance and moisture resistance, which satisfies recent requirements for high sensitivity, has not been obtained yet.
In view of the above-mentioned situation, the present invention aims at providing a novel developer capable of realizing a thermal recording material superior in preservation stability (i.e., heat resistance, moisture resistance) of color image and non-image area, which satisfies recent requirements for high sensitivity, and a thermal recording material using the same.